Conventional process heaters find wide usage in oil refineries and chemical plants and are usually fired by oil or gas. Such heaters usually consist of a refractory lined combustion chamber, fired by one or more burners, and are provided with tubular heating elements disposed within the combustion chamber. These elements consist of serpentine coils composed of vertical or horizontal tubes, spaced two or three tube diameters apart, with empty spaces in between, these allowing the back up refractory to be exposed to hot flue gas. In some instances the coils are fired from both sides, but invariably refractory surfaces exposed to hot flue gas are present. As a result, conventional heaters designed for gas or oil firing, when fired by pulverized coal, do not operate satisfactorily, because the refractories in some instances operate at temperatures above the ash fusion point, allowing molten ash particulates generated by the burning coal to rapidly accumulate on such surfaces, causing heater inoperability and forced shutdown. Impingement of molten ash on cold surfaces also poses operability and tube corrosion problems that need be considered when firing coal.
This invention seeks to overcome these problems by very nearly eliminating refractory surfaces exposed to hot flue gas and allowing cold surfaces only to be so exposed. Furthermore, provision is made to remove solidified ash particulates from cold surfaces by appropriate soot blowing equipment, while minimizing tube corrosion problems by judicious choice of construction materials. Particulate laden flue gas leaving the combustion chamber at relatively low temperature is also rendered essentially particulate free using centrifugal gas—solid separators, thereby allowing for convection preheating, using conventional bare or finned tubing, serviced by conventional soot blowing equipment, and minimizing excessive fouling that could lead to inoperability and shutdown.